Synergy is an amazingly stable aircraft. For many that’s apparent when watching various models fly. Pilots of our prototypes report that it’s like a sled on rails. But because people are more used to long-tail airplanes than the alternative physics that birds use for stability, the question comes up anyway. Synergy designer John McGinns has been granted two US Patents about creating drag reduction through increased stability and control. So let’s start from the beginning.
Synergy has four vertical tail surfaces of generous size, all intentionally loaded (flying at an angle of attack) so as to actively stabilize the aircraft in all directions, even when flying straight ahead (less hunting or tail wagging). These also direct the airflow where it will cause a drag reduction. At the wingtips, the loading counters wake vortex. At the boom tubes, the loading fills the wake efficiently. The ‘vertical tail volume coefficient*’ that results is appropriate for a pusher. Stable! (A tractor prop is destabilizing, an aft prop is stabilizing.)
However, even the need for typical yaw stabilization is already greatly reduced. Because of the downloaded elevons at the wing tips and their effect on tip vortex, Synergy is like the ultimate implementation of the Bell Shaped Lift Distribution that makes the best flying wings. Negatively loaded wing tips bring difficulties for many designs, but having the BSLD negative load placed above and behind the tips- on a controlled airfoil- well that’s just nifty. Synergy is so stable in yaw it may not even need the vertical surfaces it has.
When aircraft, such as hang gliders, have a swept wing planform like Synergy, many people recognize they’re already quite stable in pitch. Most don’t even need a tail. It all depends on the lift distribution, center of mass, and the pitching moment of the airfoils. Easy.
Straight wing airplanes, not so much. Their wings are not inherently stable about the pitch axis.
Ignoring the fuselage, then, Synergy might be regarded as a stable flying wing, in which we’ve added a very large pair of tails; flying them with an intentionally stabilizing download.
Its ‘horizontal tail volume coefficient*’ is quite conservative due to the large size and aft location of these horizontal control surfaces. (*Tail volume coefficients are numbers used to express the relative ‘aerodynamic leverage’ of a tail surface. They allow engineers to quickly compare apples to oranges among very different designs. Synergy ‘s numbers have plenty of company among noteworthy safe airplanes.)
An aeronautical design principle called decalage shows why some actually have it backwards: stability is increased in Synergy, decreased in a long-tail plane. Decalage is the difference between the angle of attack of the wing and the angle of attack of the tail. More decalage (more difference) equals more stable, less equals less stable. Synergy has more decalage.
Finally, in a long-tail airplane, the tail and the wing hit gusts at slightly different times, the delay causing the tail to amplify the resulting disturbance by inducing greater angle change. In Synergy, bumps are less amplified, and there is less ‘weathervaning’ due to crosswinds.
Prototype test pilots all agree, it’s “such a creampuff” that their only disappointment is not getting any bragging rights. One said, “I was expecting it to be more touchy, but it flies like a trainer.”
Ouch. So far, every form of analysis and testing says we have plenty of control to play with to dial it all in for our personal preferences.
Efficient Control and Stall Prevention in Advanced Configuration Aircraft, US 8657226
Aircraft Stability and Efficient Control Through Induced Drag Reduction, US 9545993
Dynamic stability depends upon a host of factors, each given equal attention, each adjustable to a degree. In the video below we stretch three seconds of flight into fifteen to illustrate the naturally damped convergent recovery due to the configuration alone.